1. Field of the Invention
The present invention relates to an edge enhancement system and method for performing edge enhancement of image signals.
2. Description of the Related Art
FIG. 5 is a block diagram of a conventional edge enhancement circuit 513. In FIG. 5, a luminance-signal generation circuit (MainY) 501 generates a main luminance signal derived from image data on which edge enhancement is to be applied. A horizontal bandpass filter (H-BPF) 502 detects a horizontal edge component of the main luminance signal generated from the luminance-signal generation circuit 501 to generate a horizontal edge signal. A gain circuit 505 applies an arbitrary gain to the horizontal edge signal generated from the horizontal bandpass filter 502 to control the amplitude of the signal.
Similarly, a vertical bandpass filter (V-BPF) 503 detects a vertical edge component of the main luminance signal to generate a vertical edge signal, and a diagonal bandpass filter (D-BPF) 504 detects a diagonal edge component of the main luminance signal to generate a diagonal edge signal. Gain circuits 506 and 507 control the amplitudes of the corresponding edge signals, respectively.
Adders 508 and 509 add the horizontal, vertical, and diagonal edge signals to generate an edge enhancement signal. A gain circuit 510 controls the amplitude of the edge enhancement signal. Finally, an adder 511 adds the main luminance signal generated from the luminance-signal generation circuit 501 to the edge enhancement signal generated from the gain circuit 510, thus generating an edge-enhanced luminance signal.
In the conventional edge enhancement circuit 513, the horizontal, vertical, and diagonal edge signals are added, thus generating a resultant edge signal. When the amplitudes of the respective edge signals are large, the amplitudes overlap each other. Thus, the amplitude of the resultant edge signal is too large. This can result in the degradation of quality of an image subjected to edge enhancement. In this instance, a spatial frequency domain with the problem of amplitude overlap resulting in abnormally large amplitude will now be described. FIG. 6 shows regions occupied by the horizontal, vertical, and diagonal edge signals in the spatial frequency domain. Referring to FIG. 6, the abscissa denotes a horizontal spatial frequency axis and the ordinate denotes a vertical spatial frequency axis. As a region comes closer to the origin, its frequency becomes lower. Referring to FIG. 6, regions H are detected by the horizontal bandpass filter 502, regions V are detected by the vertical bandpass filter 503, and regions D are detected by the diagonal bandpass filter 504. A horizontal edge signal overlaps a diagonal edge signal in the region shown by the arrow a of FIG. 6. Thus, the above-mentioned problem occurs.
FIG. 7 shows an example of the generation of a resultant edge signal having an abnormally large amplitude in the region shown by arrow a of FIG. 6. Referring to FIG. 7, the horizontal bandpass filter 502 generates a horizontal edge signal H having a waveform 701 in the region shown by the arrow a of FIG. 6. The diagonal bandpass filter 504 generates a diagonal edge signal D having a waveform 702 in the region shown by the arrow a of FIG. 6. In this case (the amplitude of a vertical edge signal is zero), as shown in FIG. 7, the maximum amplitude of a resultant edge signal Mix having a waveform 703 is too large compared to a predetermined value obtained by superimposing the amplitude of the diagonal edge signal on that of the horizontal edge signal. The disadvantage is that unnatural edge enhancement is caused by the above resultant edge signal (enhancement signal) having the abnormally large amplitude.